is it the ten gallon you have your fish in, or a new tank? if it's not seperate there isn't any sort of crab you can put in with your fish. if its seperate, I would probably go with red clawed crabs as they don't need as much land as fiddlers (that need more territory and spend much more time on land). red clawed crabs do need some land but a piece of bogwood or rock sticking out of the water will suffice. they prefer brackish water but they stay reasonably small and are more in keeping with a small tank.